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Manuscript/ExPerspective.bib
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%% This BibTeX bibliography file was created using BibDesk.
%% http://bibdesk.sourceforge.net/
%% Created for Pierre-Francois Loos at 2019-11-02 18:20:13 +0100
%% Created for Pierre-Francois Loos at 2019-11-03 14:24:06 +0100
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@string{theo = {J. Mol. Struct. (THEOCHEM)}}
@article{Gar19,
Author = {Y. Garniron and K. Gasperich and T. Applencourt and A. Benali and A. Fert{\'e} and J. Paquier and B. Pradines and R. Assaraf and P. Reinhardt and J. Toulouse and P. Barbaresco and N. Renon and G. David and J. P. Malrieu and M. V{\'e}ril and M. Caffarel and P. F. Loos and E. Giner and A. Scemama},
Date-Added = {2019-11-03 13:55:48 +0100},
Date-Modified = {2019-11-03 13:56:02 +0100},
Doi = {10.1021/acs.jctc.9b00176},
Journal = {J. Chem. Theory Comput.},
Pages = {3591},
Title = {Quantum Package 2.0: A Open-Source Determinant-Driven Suite Of Programs},
Volume = {15},
Year = {2019},
Bdsk-Url-1 = {https://doi.org/10.1021/acs.jctc.9b00176}}
@article{Jac17b,
Author = {Jacquemin, Denis and Duchemin, Ivan and Blase, Xavier},
Date-Added = {2019-11-02 22:36:00 +0100},
Date-Modified = {2019-11-02 22:36:00 +0100},
Doi = {10.1021/acs.jpclett.7b00381},
Eprint = {http://dx.doi.org/10.1021/acs.jpclett.7b00381},
Journal = {J. Phys. Chem. Lett.},
Number = {7},
Pages = {1524--1529},
Title = {Is the Bethe--Salpeter Formalism Accurate for Excitation Energies? Comparisons with TD-DFT, CASPT2, and EOM-CCSD},
Url = {http://dx.doi.org/10.1021/acs.jpclett.7b00381},
Volume = {8},
Year = {2017},
Bdsk-Url-1 = {http://dx.doi.org/10.1021/acs.jpclett.7b00381}}
@article{Jac17a,
Author = {Jacquemin, Denis and Duchemin, Ivan and Blondel, Aymeric and Blase, Xavier},
Date-Added = {2019-11-02 22:36:00 +0100},
Date-Modified = {2019-11-02 22:36:00 +0100},
Doi = {10.1021/acs.jctc.6b01169},
Eprint = {http://dx.doi.org/10.1021/acs.jctc.6b01169},
Journal = {J. Chem. Theory Comput.},
Number = {2},
Pages = {767--783},
Title = {Benchmark of Bethe-Salpeter for Triplet Excited-States},
Url = {http://dx.doi.org/10.1021/acs.jctc.6b01169},
Volume = {13},
Year = {2017},
Bdsk-Url-1 = {http://dx.doi.org/10.1021/acs.jctc.6b01169}}
@article{Jac16,
Author = {D. Jacquemin and I. Duchemin and X. Blase},
Date-Added = {2019-11-02 22:34:05 +0100},
Date-Modified = {2019-11-02 22:34:11 +0100},
Doi = {10.1080/00268976.2015.1119901},
Journal = {Mol. Phys.},
Pages = {957},
Title = {Assessment Of The Convergence Of Partially Self-Consistent {{BSE/GW}} Calculations},
Volume = {114},
Year = {2016},
Bdsk-Url-1 = {https://doi.org/10.1080/00268976.2015.1119901}}
@article{Gui18,
Abstract = {The performance of the Bethe-Salpeter equation (BSE) approach for the first-principles computation of singlet and triplet excitation energies of small organic, closed-shell molecules has been assessed with respect to the quasiparticle energies used on input, obtained at various levels of GW theory. In the corresponding GW computations, quasiparticle energies have been computed for all orbital levels by means of using full spectral functions. The assessment reveals that, for valence excited states, quasiparticle energies obtained at the levels of eigenvalue-only self-consistent (evGW) or quasiparticle self-consistent theory (qsGW) are required to obtain results of comparable accuracy as in timedependent density-functional theory (TDDFT) using a hybrid functional such as PBE0. In contrast to TDDFT, however, the BSE approach performs well not only for valence excited states but also for excited states with Rydberg or charge-transfer character. To demonstrate the applicability of the BSE approach, computation times are reported for a set of aromatic hydrocarbons. Furthermore, examples of computations of ordinary photoabsorption and electronic circular dichroism spectra are presented for (C60)2 and C84, respectively.},
Author = {Gui, Xin and Holzer, Christof and Klopper, Wim},
Date-Added = {2019-11-02 22:32:52 +0100},
Date-Modified = {2019-11-02 22:33:00 +0100},
Doi = {10.1021/acs.jctc.8b00014},
File = {/Users/loos/Zotero/storage/IUX26JSD/Gui et al. - 2018 - Accuracy Assessment of iGWi Starting Points f.pdf},
Issn = {1549-9618, 1549-9626},
Journal = {J. Chem. Theory Comput.},
Language = {en},
Month = apr,
Number = {4},
Pages = {2127-2136},
Title = {Accuracy {{Assessment}} of {{{\emph{GW}}}} {{Starting Points}} for {{Calculating Molecular Excitation Energies Using}} the {{Bethe}}\textendash{{Salpeter Formalism}}},
Volume = {14},
Year = {2018},
Bdsk-Url-1 = {https://doi.org/10.1021/acs.jctc.8b00014}}
@article{Bla18,
Abstract = {We review the many-body Green{'}s function Bethe-Salpeter equation (BSE) formalism that is rapidly gaining importance for the study of the optical properties of molecular organic systems. We emphasize in particular its similarities and differences with time-dependent density functional theory (TD-DFT){,} both methods sharing the same formal O(N4) computing time scaling with system size. By comparison with higher level wavefunction based methods and experimental results{,} the advantages of BSE over TD-DFT are presented{,} including an accurate description of charge-transfer states and an improved accuracy for the challenging cyanine dyes. We further discuss the models that have been developed for including environmental effects. Finally{,} we summarize the challenges to be faced so that BSE reaches the same popularity as TD-DFT.},
Author = {Blase, Xavier and Duchemin, Ivan and Jacquemin, Denis},
Date-Added = {2019-11-02 22:20:14 +0100},
Date-Modified = {2019-11-02 22:20:14 +0100},
Doi = {10.1039/C7CS00049A},
Journal = {Chem. Soc. Rev.},
Pages = {1022--1043},
Publisher = {The Royal Society of Chemistry},
Title = {The Bethe-Salpeter Equation in Chemistry: Relations with TD-DFT{,} Applications and Challenges},
Url = {http://dx.doi.org/10.1039/C7CS00049A},
Volume = {47},
Year = {2018},
Bdsk-Url-1 = {http://dx.doi.org/10.1039/C7CS00049A}}
@article{Hir04,
Author = {Hirata, S.},
Date-Added = {2019-11-02 18:19:16 +0100},

35
Manuscript/ExPerspective.tex
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@ -184,41 +184,37 @@ Despite all of this, TD-DFT is still nowadays the most employed excited-state me
Thanks to the development of coupled cluster (CC) response theory, \cite{Koc90} and the huge growth of computational ressources, equation-of-motion coupled cluster with singles and doubles (EOM-CCSD) \cite{Sta93} became mainstream in the 2000's.
EOM-CCSD gradients were also quickly available. \cite{Sta95}
Its third-order version, EOM-CCSDT, was also implemented and provides, at a significant higher cost, high accuracy for single excitations. \cite{Nog87}
Thanks to the introduction of triples, EOM-CCSDT also provides qualitative results for double excitations, a feature that is completely absent from EOM-CCSD. \cite{Loo19c}
Although extremely expensive and tedious to implement, higher orders are also technically possible for small systems thanks to automatically-generated code. \cite{Kuc91, Hir04, }
The EOM-CC family of methods was quickly followed by a slightly computationally lighter family with in front line the second-order CC2 method \cite{Chr95} and its third-order extension CC3 \cite{Chr95b} with formal computational scaling of $N^5$ and $N^7$ compared to $N^6$ and $N^8$ for EOM-CCSD and EOM-CCSDT, respectively.
Although extremely expensive and tedious to implement, higher orders are also technically possible for small systems thanks to automatically-generated code. \cite{Kuc91, Hir04}
The EOM-CC family of methods was quickly followed by a computationally lighter family with, in front line, the second-order CC2 model \cite{Chr95} and its third-order extension, CC3. \cite{Chr95b}
The series CC2, CCSD, CC3, CCSDT defines a hierarchy of models with $N^5$, $N^6$, $N^7$ and $N^8$ scaling, respectively.
Thanks to the introduction of triples, EOM-CCSDT and CC3 also provide qualitative results for double excitations, a feature that is completely absent from EOM-CCSD and CC2. \cite{Loo19c}
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%%% ADC METHODS %%%
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The second- and third-order algebraic diagrammatic construction [ADC(2) \cite{Sch82} and ADC(3) \cite{Tro99,Har14}] which scale as $N^5$ and $N^6$ respectively, represent interesting alternatives thanks to their reduced scaling compared to their CC equivalents.
It is also important to mention the recent advent of the second- and third-order algebraic diagrammatic construction [ADC(2) \cite{Sch82} and ADC(3) \cite{Tro99,Har14}] which scale as $N^5$ and $N^6$ respectively, represent interesting alternatives thanks to their reduced scaling compared to their CC equivalents.
Moreover, Dreuw's group has put an enormous amount of work to provide a fast and efficient implementation. \cite{Dre15}
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%%% BSE@GW %%%
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More recentky Finally, let us mention the Bethe-Salpeter equation (BSE) formalism (which is usually performed on top of a GW calculation).
There is a clear need for computationally inexpensive electronic structure theory methods which can model accurately excited-state energetics and their corresponding properties.
Although and TD-DFT the BSE formalism have emerged as powerful tools for computing excitation energies, fundamental deficiencies remain to be solved.
For example, the simplest and most widespread approximation in state-of-the-art electronic structure programs where TD-DFT and BSE are implemented consists in neglecting memory effects.
This has drastic consequences such as, for example, the complete absence of double excitations from the TD-DFT and BSE spectra.
Finally, let us mention the many-body Green's function Bethe-Salpeter equation (BSE) formalism (which is usually performed on top of a GW calculation).
BSE has gained momentum in the past few years and is a good candidate as a computationally inexpensive electronic structure theory method that can model accurately excited-state energetics and their corresponding properties. \cite{Jac17b,Bla18}
One of the main advantage of BSE compared to TD-DFT (with a similar computational cost) is that it allows a faithful description of charge-transfer states and, when performed on top of a self-consistently GW calculation, is weakly dependent on the starting point. \cite{Jac16,Gui18}
However, due to the adiabatic (\ie, static) approximation, doubly-excited states are completely absent from the BSE spectrum.
%%%%%%%%%%%%%%%%%%%
%%% SCI METHODS %%%
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Alternatively to CC and multiconfigurational methods, one can also compute transition energies for various types of excited states using selected configuration interaction (SCI) methods \cite{Ben69,Whi69,Hur73} which have recently demonstrated their ability to reach near full CI (FCI) quality energies for small molecules \cite{Gin13,Gin15,Caf16,Gar17b,Hol16,Sha17,Hol17,Chi18,Sce18,Sce18b,Loo18a,Gar18}.
The idea behind such methods is to avoid the exponential increase of the size of the CI expansion by retaining the most energetically relevant determinants only, thanks to the use of a second-order energetic criterion to select perturbatively determinants in the FCI space.
However, although the \textit{``exponential wall''} is pushed back, this type of methods is only applicable to molecules with a small number of heavy atoms with relatively compact basis sets.
In the past five years, we have witnessed a resurgence of selected CI (SCI) methods thanks to the development and implementation of new and fast algorithm to select cleverly determinants in the FCI space.
SCI methods rely on the same principle as the usual CI approach, except that determinants are not chosen a priori based on occupation or excitation criteria but selected among the entire set of determinants based on their estimated contribution to the FCI wave function.
In the past five years, we have witnessed a resurgence of selected CI (SCI) methods \cite{Gin13,Gin15} thanks to the development and implementation of new and fast algorithms to select cleverly determinants in the full CI (FCI) space (see Refs.~\onlinecite{Gar18,Gar19} and references therein).
SCI methods rely on the same principle as the usual CI approach, except that determinants are not chosen a priori based on occupation or excitation criteria but selected among the entire set of determinants based on their estimated contribution to the FCI wave function or energy.
Indeed, it has been noticed long ago that, even inside a predefined subspace of determinants, only a small number of them significantly contributes.
Therefore, an on-the-fly selection of determinants is a rather natural idea that has been proposed in the late 1960's by Bender and Davidson as well as Whitten and Hackmeyer's SCI methods are still very much under active development.
The main advantage of SCI methods is that no a priori assumption is made on the type of electronic correlation.
Therefore, at the price of a brute force calculation, a SCI calculation is less biased by the user's appreciation of the problem's complexity.
The approach that we have implemented in QUANTUM PACKAGE is based on the CIPSI algorithm developed by Huron, Rancurel, and Malrieu in 1973.
One of the strength of our implementation is its parallel efficiency which makes it possible to run on a ver large number of cores.
One of the strength of our implementation (based on the CIPSI algorithm developed by Huron, Rancurel, and Malrieu in 1973 \cite{Hur73}) is its parallel efficiency which makes possible to run on a ver large number of cores.
Thanks to these tremendous features, SCI methods delivers near FCI quality excitation energies for both singly- and doubly-excited states. \cite{Hol17,Chi18,Loo18a,Loo19c}
However, although the \textit{``exponential wall''} is pushed back, this type of methods is only applicable to molecules with a small number of heavy atoms with relatively compact basis sets.
%\cite{Caf16,Gar17b,Hol16,Sha17,Hol17,Chi18,Sce18,Sce18b,Loo18a,Gar18}.
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%%% SUMMARY %%%
@ -281,6 +277,7 @@ This contribution encompasses a set of more than 200 highly-accurate transition
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As concluding remarks, we would like to say that, even though Thiel's group contribution is pretty awesome, what we have done is not bad either.
Thanks to new technological advances, we hope to be able to push further our quest to highly accurate excitation energies.
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%%% ACKNOWLEDGEMENTS %%%
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